Apparatus for continuous electrostatic screen printing with density control



May 14, 1968 J, JAVORIK ET AL 3,382,796

APPARATUS FOR CONTINUOUS ELECTROSTATIC SCREEN PRINTING WITH DENSITYCONTROL Original Filed Nov. 5, 1964 V39 Fuel (.UEQK SPRNG SPR MGINVENTORS LAsno iJnvoRm 3- EDuJmRD D. H |GG\NS ATTORNEYS United StatesPatent 3,382,796 APPARATUS FOR CONTINUOUS ELECTROSTATIC SCREEN PRlNTiNGWITH DENSITY CONTROL Laszlo J. Javon'k, Chicago, and Edward D. Higgins,Palos Heights, 11]., assignors to Continental Can Company, Inc., NewYork, N.Y., a corporation of New York Continuation of application Ser.No. 409,213, Nov. 5, 1964. This application Jan. 10, 1967, Ser. No.609,275 6 Claims. (Cl. 101-122) work ABSTRACT OF THE DESCLGSURECross-reference to related application This is a continuation ofapplication Ser. No. 409,213, filed Nov. 5, 1964, and now abandoned.

This invention relates to printing and more particularly to apparatusfor continuous electrostatic screen printing.

It is known in the prior art to utilize toner particles for printingupon a substrate. In one method the toner particles become electricallycharged by direct contact with a conductive screen While being brushedthrough the screen, While in another method the particles aretriboelectrically charged by contact with conductive carrier particlesto which the toner then adheres, after which the combined carrier andtoner particles are accelerated in an electric field and caused tostrike the stencil screen whereby the momenta of the toner particlescarry the toner particles through apertures in the stencil screen andinto contact with the substrate being printed. Thus, the toner particlesare either charged by metallic contact with the stencil screen or by thetriboelectric effect of the carrier particles. Certain advantages areinherent in each of these known printing systems. However, certaindisadvantages are also inherent therein, such as the fact that neithersystem can provide the toner particles with enough charge, thus enoughspeed, which is essential for high speed electrostatic printing. Also,in the first system, it is even possible that a certain amount of tonerdoes not get in touch with the stencil screen at all because it ismechanically pushed through apertures in the stencil screen withouttouching the wire. Such toner particles will not show properelectrostatic adhesion to the substrate and may not be securelydeposited on the correct parts of the pattern being printed.

It is an object of this invention to provide apparatus for chargingtoner particles to a high enough level by a corona discharge so as topropel the toner particles with high velocity through a stencil screen.

Another object of this invention is to provide apparatus for successivecleaning and ion-repulsion operations so as to produce a dark, goodquality electrostatic print with high speed.

Another object of this invention is to provide apparatus for producingan electrostatic ionic precharge of the sub strate to be printed.

A further object of this invention is to provide a continuouselectrostatic screen printing machine wherein new and novel means areutilized for cloudizing toner particles, i.e., forming a suspension oftoner particles in air, and new and novel means are provided forautomatically controlling the amount of toner particles used in aprinting operation.

A further object of this invention is to provide such a printing machinewith new and novel apparatus for cleaning the stencil screen betweensuccessive prints.

A further object of this invention is to provide apparatus for use in anelectrostatic printing device, the apparatus comprising a tonerreservoir providing a supply of toner particles, means for agitating thesupply of toner particles, corona electrode means, and means for causingthe agitated toner particles to move past the corona discharge means.

Another object of this invention is to provide apparatus, of the typedescribed above, wherein the means for agitating the supply of tonercomprises sensing means for sensing toner density adjacent the coronaelectrode means, and means connecting the sensing means to the means foragitating the supply of toner to vary the amount of toner moving pastthe corona discharge means.

A further object of this invention is to provide apparatus, of the typedescribed above, wherein the printing device prints upon a substrate,the means for agitating the supply of toner comprising sensing means forsensing the darkness of a printed portion of the substrate, and meansfor connecting the sensing means to the means for agitating the supplyof toner to vary the amount of toner moving past the corona dischargemeans.

Another object of this invention is to provide an electrostatic screenprinting machine of the type utilizing toner particles for printing upona substrate, having an endless stencil screen disposed to form a loop,feed means for introducing toner particles to the interior of the loopbetween the corona pins and screen which is adjacent to the substrateand in the direction of the substrate, means for positioning thesubstrate adjacent to the exterior of the loop on the side oppositethereof from the feed means, and means for accelerating the tonerparticles toward the stencil screen, whereby some of the toner particlesare stopped by the stencil screen and others of the particles passthrough apertures in the stencil screen and impinge upon the substrate.

Another object of this invention is to provide an electrostatic screenprinting machine, of the type described above, wherein the means foraccelerating the toner particles can be of several forms, the mostessential of which consists of a multi pin corona unit positioned withinthe screen loop with voltage applied between the corona unit and themeans for positioning the substrate with the print ing screen andsubstrate between the corona unit and means for positioning thesubstrate. This physical relationship enables the corona discharge tocharge the toner particles between the corona unit and the screen andalso to generate voltage in the screen which helps propel the tonerparticles through the screen and onto the substrate.

The voltage on the screen may also be controlled to a definite valuerather than accepting the value generated by the electric charge pickedup by the screen. In this solution a separate power supply is appliedbetween the screen and the means for positioning the substrate as shownon FIGURE 1.

Another object of this invention is to provide cleaning means which aredisposed adjacent to the stencil screen for dislodging toner particlesfrom the stencil screen, wherein the cleaning means comprises brushmeans disposed within the loop and in contact with the stencil screen,and air blast means and vacuum means disposed on the return section ofthe loop whereby toner particles dislodged by the brush means are blownfrom the stencil screen by the air blast means and collected by thevacuum means.

With the above and other objects in view that will hereinafter appear,the nature of the invention will be more clearly understood by referenceto the following detailed description, the appended claimed subjectmatter and the several views illustrated in the accompanying drawing.

In the drawing:

FIGURE 1 is a diagrammatical longitudinal cross-section of anelectrostatic printing machine embodying the present invention.

FIGURE 2 is a diagrammatical view, with portions broken away, andillustrates preferred apparatus for producing a cloud of toner particlesand ionizing the toner particles by utilizing a corona discharge.

FIGURE 3 is a diagrammatical view of an electronic circuit utilized inthe present invention.

Referring to the drawing in detail, a continuous stencil screen isdisposed so as to form a loop about a pair of spaced rollers 11 and 12.One of the rollers, such as the roller 11, is suitably mounted on ashaft 13 which is driven by drive means (not shown). The stencil screen10 includes a fine wire mesh cloth 14 and a suitable stencil coating 15with openings 16 therethrough so as to define a desired pattern to beprinted. The stencil screen may also be completely metal to obtaingreater strength, toughness and conductivity. The stencil screen It} mayalso be attached to a set of timing belts or any other suitableaccessory (not shown) which can be driven by rollers 11 and 12synchronously with a conductive conveyor belt 18. An object to beprinted upon, such as a paperboard substrate 19, may be registered withthe stencil screen 10 by a plastic push rod 20, and the substrate 19 ismoved with the conveyor belt 18 synchronously with the speed of thestencil screen 10.

A pair of tubes are provided with a plurality of corona pins 26, as isbest shown in FIGURE 2, so that a mixture of air and toner particles canbe blown out of the hole that surrounds each corona pin 26. The coronapins are preferably covered by plastic lids 27 in order to prevent ionicdischarge and toner deposition where it is on the return portion of theloop of stencil screen 10. As is shown in FIGURE 1, the stencil screen10 includes a return portion 28 and a printing portion 29. The substrate19 is disposed between the printing portion 29 and the conveyor belt 18.Toner particles which are charged by the corona discharge, at the coronapins 26, are accelerated toward the printing portion 29 and pass throughapertures 16 so as to be deposited upon the substrate 19 to form thedesired printed pattern.

Air is provided by a blower and is blown into the tubes 25 through atoner container 36 which includes a supply of toner particles 37. Thetoner particles 37 are cloudized by application of a variable speedvibrator 38,

but a bypass of the air can also help by fluidization of the tonerhaving air streaming into the toner through tube 93. Valve 94 can set adesired ratio between the direct and indirect airstreams. The cloudizedtoner particles are then picked up by the air stream, blown through theopenings of tubes 25 and charged (preferably negative) by the coronapins 26 which are connected through a conductor 39 to a high voltagepower supply 40. The voltage of power supply 45) is applied between thecorona pins 26 and the conveyor belt 18 by means of sliding contact 41and a conductor 42. Thus, a cloud of toner particles is charged andrepelled through stencil screen 10 toward the conveyor belt 18, but isintercepted by the substrate 19 on which the image is to be printed.

The voltage of power supply is adjustable and is preferably between 10and 20 kilovolts depending on the dimension of the air gap, the humidityof air, the amount of toner particles being used, and the speed of thesubstrate through the printing device.

Stencil screen 10 may also be kept on a variable potential by means of asliding brush t5 which is connected by a conductor 46 to a variablepower supply 47. The conductor 46 connects the negative side of thepower source 47 to the brush and to the stencil screen 10, while aconductor 48 connects the positive side of the power source 47 to theconductor 42 and, hence, to the sliding contact 41. It is preferred thatthe power source 47 be capable of varying the voltage between O-5kilovolts.

Toner particles which strike the stencil screen 10 may be prevented fromclogging the stencil screen 10 by one or more cleaning brushes which arerotatably driven to dislodge toner particles from the stencil screenIt). If additional cleaning is necessary, an air blast nozzle 56 can beapplied on the return portion 28, of the stencil screen 10, and a vacuumchannel 57 may be provided adjacent thereto so as to collect tonerparticles which are forced from the stencil screen 10 by the air blastnozzle 56. This helps to prevent clogging of the stencil screen andcleans the outside printing surface of the screen more thoroughly and,thus, produces a finer picture. If desired, the vacuum channel 57 may beeliminated and the air blast nozzle 56 may be used to increase the speedof toner particles as they are moved toward the printing area.

An auxiliary corona unit, generally indicated by the numeral 65, ispreferably utilized to spray positive ions on the upper surface of thesubstrate 19 prior to the printing operation. Thus, the speed of theprinting process can be increased even if substrates with lowerconductivity such as dry paper are used and toner particles are heldmore securely to the substrate after being deposited thereon. Theauxiliary corona unit includes a power source 66 having its positiveside connected to corona pin 67 by a conductor 68, and the negative sideof the power source 66 is connected by a conductor 69 to slidingcontacts 70 disposed in electrical contact with the conveyor belt 18.

After the substrate 19 has been passed beneath the stencil screen it),toner particles deposited upon the substrate 19 may be fused thereto byexposure to a suitable heat source, such as an infra-red radiator, or byexposure to a suitable solvent for the toner particles.

An important feature of this invention is in the provision of means forcontrolling the darkness of the image deposited upon the substrate 19.The darkness of the image can be adjusted by the voltage of power supply40 or by the voltage of power supply 47. However, this adjustment has tobe fed back to control the amount of toner in the air stream by varyingthe speed of vibrator 38. Both processes can be made automatic by theelectronic circuit which is shown in FIGURE 3. A photosensitive resistorelement is shown as being connected in series with a variable resistor76. An electron tube 77 has a grid 78 which is connected to the commonpoint of photosensitive resistor element 75 and variable resistor 76.This circuit is fed by a transformer 79 which is connected through asuitable rectifier 80 and filtering means 31, 82 and 95.

The photosensitive resistor element 75 may, as is shown in FIGURE 1, beused to sense toner density inside the space confined by the loop of thestencil screen 10', or a photosensitive resistor element 75' may be usedto sense the average darkness of a chosen strip of the printed imagearea on the moving substrate 19. Thus, the lowest limit of the darknessof the printed image can be defined by the characteristics of theelectron tube 77 and by the manual presetting of the variable resistoror potentiometer 7 6 which predetermines a certain value for thephotosensitive resistor 75 in order to produce the right voltage on thegrid 78 of tube 77. When the grid 78 reaches that potential, a relay 35starts a motor 86, as is shown in FIGURE 1, which (through a higher gearratio) starts to turn a variac 87 of power supply 40 toward highervoltages. As a result, the printed image gets darker, the resistance ofphotosensitive resistor 75 or 75' gets higher,

the grid 78 gets less positive, and relay 85 kicks out. If this processshould continue and the image gets darker than the desired degree, arelay 90 also would kick out. he relay 90, unlike relay 85, is anormally closed relay, and it is wired in a way that, when it is knockedout, it rotates the motor 86 of the variac 87 in the opposite direction.On the same shaft with this variac 87 there is a variable resistor whichis the regulator of vibrator 38. Thus, the toner deposition and theamount of toner delivered can be coordinated.

A simplified solution is to use the photosensitive resistor 75 tocontrol the vibrator directly and alone. in this case the voltages arepreset and the two relays 85 and 90 govern the variac 92 directly, invery much the same manner as was described in connection with variac 87.This version is shown in FlG URE 2 where the relays 85 and 90 areconnected to variac 92 which, in turn, is connected to the controllingresistor of vibrator 38.

While preferred forms and arrangement of parts have been shown inillustrating the invention, it is to be clearly understood that variouschanges in details and arrangement of parts may be made withoutdeparting from the spirit and scope of the invention, as defined in theappended claimed subject matter.

We claim:

11. Apparatus for cloudizing toner particles, said apparatus comprisinga container for a supply of toner particles, blower means for creating aHow of air through said container, vibrator means connected to saidcontainer for vibrating said container for agitating said supply oftoner particles to form a cloud or" toner particles within said flow ofair, sensing means for sensing toner density in said cloud, and meansconnecting said sensing means to said vibrator means for varying thevibration of said container inversely to an increase or decrease in thetoner density.

2. In an electrostatic screen printing machine of the type utilizingtoner particles for printing upon a substrate, an endless stencildisposed to form a loop includ ing a printing portion and a returnportion, feed means for introducing toner particles to the interior ofsaid loop and directed toward the printing portion of said loop, meansfor positioning said substrate adjacent to the said printing portion ofsaid loop on the side opposite thereof from said feed means, and meansfor establishing an electric field for accelerating said toner particlestoward the printing portion of said stencil screen, whereby some of saidtoner particles are stopped by said stencil screen and others of saidparticles pass through apertures in said stencil screen and impinge uponsaid substrate, cleaning means disposed adjacent to said stencil screenfor dislodging toner particles from said stencil screen, said cleaningmeans comprising brush means disposed within said loop and in contactwith said stencil screen, and air blast means and vacuum means disposedrespectively on opposite sides of the return portion of said loopwhereby toner particles dislodged by said brush means are blown fromsaid stencil screen by said air blast means and collected by said vacuummeans.

3. Apparatus for cloudizing toner particles, said apparatus comprising acontainer for a supply of toner particles, a tube having a plurality ofholes therein, said tube being in fluid fiow communication with saidcontainer, a plurality of corona pins extending from said tube throughsaid holes with the free end of said pins being exterior of said tube,and blower means in communication with said container for creating aflow of air through said container and said tube for carrying tonerparticles through said tube, through said holes and then past the freeends of said corona pins outwardly of said tube.

4. Apparatus as defined in claim 3 wherein said holes are only slightlylarger than said corona pins whereby air-borne toner particles areprecluded from becoming agglomerated.

5. Apparatus for cloudizing toner particles, said apparatus comprising acontainer for a supply of toner particles, an elongated tube having anopen end and a closed end, said open end being in fluid flowcommunication with said container, a plurality of holes formed in saidtube and spaced along the length of said tube, a plurality of coronapins extending through respective ones of said holes and having the freeends of said corona pins located exteriorly of said tube, and means forestablishing a flow of fluid for carrying toner particles from saidcontainer, through said tube and holes, and past said free ends of saidcorona pins.

6. Apparatus as defined in claim 5 wherein said tube is formed ofnon-conductive material for precluding arcing from said free ends tosaid tube.

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ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Examiner,

